Temperature indicating system



Feb. 2, 1937. F. s. STICKNEY TEMPERATURE INDIGATING SYSTEM Filed July 51, 1935 t INVENTOR frna/dfi/f hneg.

WITNESSES:

ATT EY Patente :1 Feb. 2, 1937 UNITED STATES PATENT OFFICE I 2,069,509 TEMPERATURE INDICATING SYSTEM Pennsylvania Application July 31, 1935, Serial No. 34,014

3 Claims.

My invention relates to temperature indicating systems and particularly to such systems in which a single electrical measuring instrument is arranged to indicate the temperature at any of a 5 number of points by means of a plurality of temperature responsive elements, such as search resistors, located at the various points where temperatures are to be ascertained.

In such systems it is usual to provide a Wheatstone bridge circuit having a pair of diagonally opposite terminals connected to a suitable source, and the remaining pair of diagonally opposite terminals connected to the sensitive element of the indicating instrument. some form of switch is provided for switching the search coils successively into one arm of the bridge circuit, so that an individual reading of the instrument may be taken with each search coil in circuit. In order to respond to the very small voltage difierences existing across the output terminals of the bridge, the indicating instrument must be quite sensitive. If the switch is arranged to open the bridge circuit entirely in changing from one search coil to another, a large part of the total voltage of the source will be impressed upon the sensitive element of the indicating instrument with consequent likelihood of damage to the instrument. In order to avoid this difiiculty, it is usual to provide a switch having overlapping contacts so that when the search resistors are changed, the bridge circuit will not be open-circuited.- However, during the transition period when two search coils are connected in the bridge circuit by the overlapping contact arrangement, the resistance of the corresponding arm of the bridge is approximately cut in half, and an undesirably large voltage is impressed upon the sensitive element of the indicating instrument.

In the systems of the prior art, the indicating element is usually a single coil device connected across a diagonal of the bridge circuit. Because of this connection, the current in the instrument .oil is proportional to line voltage and any change of line voltage produces an error in the temperature reading.

In accordance with my invention I avoid the difiiculties mentioned above by providing a second switch in series with the source and the input terminals of the bridge circuit. This switch is preferably interlocked with the switch for transferring the search resistors in and out of the bridge circuit, in such manner that the input terminals of the bridge circuit cannot be connected to the source, so long as the bridge circuit itself includes an open connection. Al-

though there are a number of switch devices suitable for carrying out the purpose of my invention, I prefer to use plug and jack devices similar to those used in telephone circuits. I also provide a crossed-coil indicating instrument in place of the usual single coil instrument, and connect the crossed-coil instrument in a novel manner such that the effect of line voltage variations on the temperature readings is substantially eliminated.

It is accordingly an object of my invention to provide a novel temperature indicating system' which will be free from errors caused by line voltage variations, within reasonable limits.

Another object of my invention is to provide a novel temperature indicating system having a Wheatstone bridge circuit and an indicating instrument energized from the bridge circuit, which will be provided with means for preventing the energization of the instrument when the bridge is in a highly unbalanced condition.

Other objects of my invention will become evident from the following detailed description taken in connection with the accompanying drawing in which the single figure is a diagrammatic view of a temperature indicating system embodying my invention.

Referring to the drawing in detail, a Wheatstone bridge circuit 5, having input terminals i and i and output terminals 2 and 3, is arranged to energize a suitable indicating instrument 6.

The bridge circuit 5 consists of a pair of equal resistors 'I, a resistor B and any one of a plurality of search resistors 9, which may be located at suitable points where temperature conditions are to be ascertained, such as in the windings or bearings of electrical machines, or in the diiierent rooms of a building. The resistor 8 has approximately the same resistance value as one of the search resistors 9.

One terminal of each of the'search resistors 9 is connected to the bridge circuit 5 by means of a series conductor ill and a compensating conductor H. The remaining terminal of each of the search resistors 9 is connected to a terminal 2 of the bridge circuit 5 by any suitable means, such as a common return conductor H. The compensating conductors l l have approxiimately the same resistance and temperature coeificient as the common conductor l2 or other return conductor. With this arrangement, when the compensating conductor I l is connected in one arm of the bridge circuit, and the corresponding search resistor 9 and the common return conductor H are connected in an adchanges in the conductor I2 produced by ambient temperature variations are neutralized by. the

similar resistance changes producedin the compensating conductor ll. g 7

The instrument 9 is preferably of the cross coil type, shown diagrammatically in the figure, comprising a stationary permanent magnet 13 and a movable armature positionedto be'infiuenced by the field of the latter. The armature of the.

instrument 9 includes a pair of coils l4 and I5 mounted at an angle with respect to each other.

The coil I4 is connected in series with an ad-- iusting resistor l9 and the input terminals I and l of the bridge circuit 5 to a suitable source of direct current, indicated as a pair of supply conductors LI and L2. The coil I5 is connected across the output terminals 2 and 9 of the bridge circuit 5, in series with an adjustable resistor 2|, to respond to the difference of voltage existing across these terminals during unbalanced resistance conditions of the bridge circuit 5. The instrument 6 is preferably calibrated in temperature units, such as degrees centigrade, in order to give direct temperature readings. Terminal 4 of the bridge circuit 5 is connected to a telephone plug it which may be inserted in any of a number of jacks l'l, corresponding to the search resistors 9. Each of the jacks II includes-a pair of normally open contacts l9, which can be closed only when the plug [6 has been inserted in the jack and has previously made contact with a spring contact member l9. The

contacts i 9 are arranged to connect the corre--- able points in the rooms,-in the usual manner.

An adjusting resistor 20 is included in series with each compensating conductor II, in order to provide an adjustment for minor irregulari-i ties of resistance when the apparatus is installed.

-, Preliminary to operations of the system, the

resistors l9 and 2| are adjusted to bring the instrument deflection within the proper scale range, and the resistors 29 are adjusted so that the bridge 5 is balanced at a predetermined balance temperature of the search resistors 9.

The operation of the above described apparatus may be set forth as follows: with the plug 22in the upper jack l1, as shown, the search resistor 9 at the extreme right of the figure is connected in the bridge circuit, and, a voltage is impressed on the coil l5, dependent upon the degree of unbalance existing in the bridge circuit.

The coil ll of the instrument 6 produces a fiux component proportional to the line voltage, and the coil l5 produces a flux component proportional to the difference of voltage existing across the output terminal of the bridge circuit I. The resultant of these two flux components causes the armature of the instrument 9 to rotate to a position in which the resultant lines up with the magnetic field produced by the permanent magnet I}. As both fiux components depend upon 4'; iacent arm of the bridge circuit, the resistance ment armature depends only upon e aflve.

degrees.

1: the plug zz a withdrawn; Jack H, the contacts ll'are'first: before the plugi22 leaves thev spring contact member l9 of the fiux'resultant with lama-1a The separation'of the contacts I. interruptsthe I energizing circuit for the bridge circuit i, so

that during the transitionperiodno voltage is impressed upon the indicating instrument 9. As the withdrawing movement of the plug" is continued, the tipof the latter finally separates I from the contact member l9, thereby interrupting the connection of the corresponding search coil 9 in the bridge circuit.

If the plug 22 is inserted in another Jack I], the sequence of connections is such that the mesh circuit of the bridge consisting of the resistors 1, 8 and 9 is completed before any voltage is applied across the input terminals of the mesh. In this way, the application of voltage to the indicating instrument 6' is prevented except at such times as the mesh is complete and in approximately balanced condition.

I do not intend that the present invention shall be restricted to the specific structural details, arrangement of parts, or c rcuit connections herein set forth, as various modifications thereof may be effected without departing from the spirit and scope of my invention. I desire therefore that only such limitations shall be imposedas areindicated in the appended claims.

c I claim as my invention:

1. In temperature indicating apparatus, a source of electric power, an elec'tricalindicating instrument, energizing means for said instrument including a Wheatstone bridge circuit having a first terminal connected to said source, second and third terminals connected to said instrument, and a fourth terminal, a plurality of search resistors at different locations, conductors for said search resistors including a series conductor and a compensating conductor connected to a common terminal of each of said search resistors, and conducting means connecting the remaining terminal of each of said search resistors to said second terminal of said bridge, a plug connected to said fourth terminal of said bridge, and a jack for each of said search resistors, each of said jacks including contact means effective upon insertion of said plug to connect said fourth terminal oi said bridge to the corresponding compensating conductor and i I ment including a Wheat'stone bridge circuit having a first terminal connected to said source,

second and third terminals connected to said instrument, and a fourth terminal, a plurality of search resistors at different locations, conductors for said search resistors including a series conductor and a compensating conductor connected to a common terminal of each of said search resistors, and conducting means connecting the remaining terminal of each of said search resistors to said second terminal of said bridge, said compensating conductors each having sub- I stantially the same resistance and substantially the same temperature coeflicient of resistance as said conducting means, a. plug connected to said fourth terminal of said bridge, and a jack for each of said search resistors, each of said jacks including contact means effective upon inser-' tion of said plug to connect said fourth terminal of said bridge to the corresponding compensat-' 20 terminals connected to said first internal circuits, and a fourth terminal, said second internal circuit being connected in series with source and said first and fourth terminals, a plurality of search resistors at difierent locations, conductors for said search resistors including a series conductor and a compensating conductor connected to a common terminal of each of said search resistors, and conducting means connecting the remaining terminal of each of said search resistors to said second terminal of said bridge, a plug connected to said fourth terminal of said bridge, and a jack for each of said search resistors, each of said jacks including contact means effective upon insertion of said plug to connect said fourth terminal of said bridge to the corresponding compensating conductor and thereafter connect the corresponding series conductor to said source.

FERNALD S. STICKNEY. 20 

